1. Field of the Invention
The present invention relates to a semiconductor device with a thin film transistor (hereinafter referred to as TFT) formed of a semiconductor film that has a crystal structure and is formed on a substrate, and to a method of manufacturing the semiconductor device.
2. Description of the Related Arts
A display for displaying text and images is indispensable means for people to digest information with various semiconductor devices that have semiconductor elements, such as television sets, personal computers, and cellular phones. CRTs have long been in the market to acquire the position of representative display. On the other hand, liquid crystal displays and other flat displays (flat panel displays) are lately increasing their shares exponentially, for electronic devices are desired to reduce weight and size.
One mode of flat panel displays is active matrix driving in which a TFT is provided in each pixel or dot and data signals are sequentially written to display an image. A TFT is an indispensable element in active matrix driving.
Most TFTs are formed from amorphous silicon. Those TFTs cannot operate at high speed and therefore they are used only as switching elements provided in respective dots. Since the TFTs cannot make other elements than switching elements, external ICs (driver ICs) mounted by TAB (tape automated bonding) or COG (chip on glass) are used in data line side driving circuits for outputting video signals to data lines and in scanning line side driving circuits for outputting scanning signals to scanning lines.
However, mounting a driver IC is considered as a limited method because the pixel pitch is reduced as the pixel density is increased. For instance, at a pixel density of UXGA level (1200×1600 pixels) in the RGB color method, at least 6000 connection terminals are necessary even by a crude estimation. An increase in number of connection terminals leads to increased occurrence of contact failure. It also leads to an increase in area of the border surrounding the pixel portion (called a picture frame region), which hinders reduction in size of a semiconductor device that employs this display and spoils the external design of the semiconductor device. Against this background, apparently a display device in which driving circuits are integrated with a pixel portion is needed. By integrally forming a pixel portion and a scanning line side and data line side driving circuits on the same substrate, the number of connection terminals can be markedly reduced as well as the area of the picture frame region.
The integrated driving circuits are demanded to have high driving performance (ON current: Ion) and to improve their reliability by preventing degradation due to the hot carrier effect whereas low OFF current (Ioff) is required for the pixel portion. A lightly doped drain (LDD) structure is known as a TFT structure capable of reducing the OFF current value. In the LDD structure, an LDD region, which is lightly doped with an impurity element, is placed between a channel formation region and a source region or drain region heavily doped with an impurity element. A structure that is known to be effective in preventing degradation of ON current value due to hot carriers is an LDD structure in which an LDD region partially overlaps a gate electrode (gate-drain overlapped LDD; hereinafter abbreviated as GOLD).
A TFT is manufactured by layering a semiconductor film and an insulating film or conductive film while using photo masks to etch the films into given shapes. If optimization of TFT structures to suit what are demanded for the pixel portion and the driving circuits is dealt with by simply increasing the number of photo masks, the manufacture process becomes complicated and the number of steps is increased inevitably.